It is of importance to have knowledge of the transmission properties of fibres in the cables included in optical transmission systems. This primarily applies to the attenuation of the fibres and the widening, so-called dispersion, which occurs when transmitting the light pulses. The total dispersion is mainly composed of two components, material dispersion and mode dispersion. Of these components, mode dispersion gives rise to uncertainty for varying fibre length, since it cannot be accurately forecast theoretically, even if the value of the respective component of the dispersion is known for a given fibre length.
It is previously known, e.g. from Ericsson Review No. 3, 1980 "Transmission mesurements on fibre cables" to use a measuring object, i.e. a given length of the fibre desired to use as a transmission fibre. The fibre transmission function H(f) is determined by measuring input and output signals at certain equidistant points. By determining this function, the inpulse response can then be determined and thereby the dispersion. The method requires time-consuming measurements and expensive instruments, which are often not suitable for use in the field and often lack the necessary dynamics.